Large scale commercial manufacturing and refining processes typically use a process controller to control the operation of one or more process control devices such as valve mechanisms which, in turn, control one or more process variables comprising, for example, fluid flow, temperature, pressure, etc., within the process. Generally, each process control device has an actuator controlled by a positioner that moves an associated control element, such as a valve plug, a damper, or some other alterable opening member in response to a command signal generated by the process controller. The control element of a process control device may, for example, move in response to changing fluid pressure on a spring biased diaphragm or a piston or in response to the rotation of a shaft, each of which may be caused by a change in the command signal. In one standard valve mechanism, a command signal with a magnitude varying in the range of 4 to 20 mA (milliamperes) causes a positioner to alter the amount of fluid and thus, the fluid pressure, within a pressure chamber in proportion to the magnitude of the command signal. Changing fluid pressure in the pressure chamber causes a diaphragm to move against a bias spring which, in turn, causes movement of a valve plug coupled to the diaphragm.
Process control devices usually develop or produce a feedback signal indicative of the response of the device to the command signal and provide this feedback signal (or response indication) to the process controller for use in controlling a process. For example, valve mechanisms typically produce a feedback signal indicative of the position (e.g., travel) of a valve plug, the pressure within a fluid chamber or the value of some other phenomena related to the actual position of the valve plug.
While a process controller generally uses these feedback signals, along with other signals, as inputs to a highly tuned, centralized control algorithm that effects overall control of a process, it has been discovered that poor control loop performance may still be caused by poor operating conditions of the individual control devices connected within the control loop. In many cases, problems associated with one or more of the individual process control devices cannot be tuned out of the control loop by the process controller and, as a result, the poorly performing control loops are placed in manual or are detuned to the point where they are effectively in manual. The processes associated with these control loops require constant supervision by one or more experienced human operators, which is undesirable.
Poor control loop performance can usually be overcome by monitoring the operational condition or the "health" of each of the process control devices connected within the loop, or at least the most critical process control devices connected within the loop, and repairing or replacing the poorly performing process control devices. The health of a process control device can be determined by measuring one or more parameters associated with the process control device and determining if the one or more parameters is outside of an acceptable range.
One process control device parameter that may be used to determine, and that is indicative of the health of a process control device is dead band. Generally speaking, in process instrumentation, dead band is the range through which an input signal may be varied, upon reversal of direction, without initiating an observable change in an output signal. Dead band, which may be caused by the physical play between mechanically interconnected components, friction, and/or other known physical phenomena, is best observed when a command signal causes a reversal in the direction of movement of a moveable element of a process control device. During this reversal, the command signal undergoes a discrete amount of change (dead band) before the movable element of the process control device actually exhibits movement in the new direction. Put another way, the difference between the value of the command signal (or other control signal) at which movement of the process control device element in a first direction last occurred and the value of the command signal (or other control signal) at which the movement of the process control device element first occurs in a second and different direction is a measure of the dead band of the process control device.
Other device parameters that may be used to determine the health of a process control device are dead time and response time. Dead time is associated with, and may be considered to be a measurement of the amount of time it takes the process control device to actually begin moving a moveable element in response to a change in a control signal. Response time is the amount of time it takes the moveable element of a process control device to reach a certain percentage, for example 63 percent, of its final value in response to a change in a control signal.
If the dead band, dead time, response time, or other process control device parameter(s) of a process control device increase a significant amount over their calibrated values, it may be necessary to repair or replace the process control device to establish adequate control within the process control loop. However, it is not usually very easy to measure process control device parameters, such as dead band, dead time, and response time to monitor the health of functioning process control devices when those devices are connected on-line within a control loop.
In the past, operators have had to remove a process control device from a control loop to bench test the device or, alternatively, control loops have been provided with bypass valves and redundant process control devices to make it possible to bypass a particular process control device to thereby test that device while the process is operating. Alternatively, operators have had to wait until a process is halted or is undergoing a scheduled shut-down to test the individual process control devices within the process. Each of these options is time consuming, expensive, and still only provides intermittent measurement of the parameters of the individual process control devices required to determine the health of those control devices.